1. Field of the Invention
The present invention relates to the construction of an electro-optical device. The present invention particularly relates to an active matrix type electro-optical device having a thin film transistor (TFT) made on an insulating substrate.
2. Related Background Art
In recent years, an EL display has been attracting attentions as a flat panel display that is to replace an LCD (liquid crystal display), and is actively researched. In the specification, the EL display has an EL element which is also called a light emitting device or a light emitting diode. Further, the EL (Electro Luminescence) includes triplet-based light emission or singlet-based light emission.
There are generally two types of driving system for the LCD display. One type is a passive matrix type used in an STN-LCD, etc. The other type is an active matrix type used in a TFT-LCD, etc. Similarly, there are generally two kinds of driving systems in the EL display. One type is a passive matrix type, and the other type is an active matrix type.
In the case of the passive matrix type, a wiring to serve as an electrode is arranged in each of upper and lower portions of an EL element. A voltage is sequentially applied to the wirings, and an electric current flows through the EL element so that the EL element is lighted.
In contrast to this, in the case of the active matrix type, each pixels has a TFT, and a signal can be held within each pixels.
FIGS. 15A and 15B show a constructional example of the active matrix type electro-optical device used in the EL display. FIG. 15A is a view showing the construction of the entire circuit in which a pixel portion is arranged in the center of this circuit. A gate signal line side driving circuit for controlling the operation of a gate signal line is arranged to the left of the pixel portion. A source signal line side driving circuit for controlling the operation of a source signal line is arranged above the pixel portion. In FIG. 15A, a portion surrounded by a dotted line frame shows a circuit of one pixel. FIG. 15B shows an enlarged view of this circuit. In FIG. 15B, reference numeral 1501 designates a TFT (hereinafter called a switching TFT) functioning as a switching element when a signal is written into a pixel. In FIG. 15B, the switching TFT has a double gate structure, but may also have a single gate structure, a triple gate structure or a multi-gate structure having more than three gates. One of polarities of the TFT may be selected in accordance with a constructional form of the circuit. Reference numeral 1502 designates a TFT (hereinafter called an EL driving TFT) functioning as an element (an electric current control element) for controlling an electric current supplied to an EL element 1503. In FIG. 15B, the TFT 1502 is arranged between an anode 1509 of the EL element 1503 and an electric current supply line 1507. In an alternative constructional method, it is also possible to arrange the TFT 1502 between a cathode 1510 of the EL element 1503 and a cathode electrode 1508. One of polarities of the TFT may be selected in accordance with the constructional form of the circuit. In this case, a system is common and often used in which a p-channel type TFT is used for the EL driving TFT, and the EL driving TFT is arranged between the anode 1509 of the EL element 1503 and the electric current supply line 1507, since source grounding is preferable as the operation of a transistor, and there is a restriction in manufacture of the EL element 1503. Reference numeral 1504 designates a holding capacitor for holding a signal (voltage) inputted from a source signal line 1505. One terminal of the holding capacitor 1504 in FIG. 15B is connected to the electric current supply line 1507, but there is also a case in which dedicated wiring is used. A gate terminal of the switching TFT 1501 is connected to a gate signal line 1506, and a source terminal of this TFT 1501 is connected to the source signal line 1505. A drain terminal of the EL driving TFT 1502 is connected to the anode 1509 of the EL element 1503, and a source terminal of this TFT 1502 is connected to the electric current supply line 1507.
An operation of the circuit of the active matrix type electro-optical device will next be explained with reference to FIGS. 15A and 15B. First, when the gate signal line 1506 is selected, a voltage is applied to a gate of the switching TFT 1501, and the switching TFT 1501 attains a turned-ON state. Thus, a signal (voltage) of the source signal line 1505 is accumulated in the holding capacitor 1504. The voltage of the holding capacitor 1504 becomes a voltage VGS between the gate and the source of the EL driving TFT 1502 so that an electric current according to the voltage of the holding capacitor 1504 flows through the EL driving TFT 1502 and the EL element 1503. As a result, the EL element 1503 is lighted.
Luminance of the EL element 1503, i.e., an electric current amount flowing through the EL element 1503 can be controlled by VGS. VGS is the voltage of the holding capacitor 1504, and is a signal (voltage) inputted to the source signal line 1505. Namely, the luminance of the EL element 1503 is controlled by controlling the signal (voltage) inputted to the source signal line 1505. Finally, the gate signal line 1506 is set to a not-selected state, and the gate of the switching TFT 1501 is closed, and the switching TFT 1501 is set to a turned-OFF state. At that time, electric charges accumulated in the holding capacitor 1504 are held. Accordingly, VGS is held as it is, and an electric current according to VGS continuously flows through the EL driving TFT 1502 and the EL element 1503.
The descriptions above are reported in SID99 Digest: P372: xe2x80x9cCurrent Status and future of Light-Emitting Polymer Display Driven by Poly-Si TFTxe2x80x9d, ASIA DISPLAY 98: P217: xe2x80x9cHigh Resolution Light Emitting Polymer Display Driven by Low Temperature Polysilicon Thin Film Transistor with Integrated Driverxe2x80x9d, Euro Display99 Late News: P27: xe2x80x9c3.8 Green OLED with Low Temperature Poly-Si TFTxe2x80x9d, etc.
In the active matrix type electro-optical device, it is required that the pixel has a large holding capacity and high aperture ratio in view of display performance of this device. Since each pixel has the high aperture ratio, utilization efficiency of light is improved and a display unit can be saved in power and made compact.
In recent years, the pixel is reduced in size and an image with higher definition is required. Since the pixel size is reduced, regions for forming the TFT and wiring come to occupy increased area in one pixel, and the aperture ratio of the pixel is reduced.
Therefore, efficient layout of circuit elements that are required in the circuit construction of the pixel is indispensable to obtain a high aperture ratio of each pixel in the prescribed pixel size.
As mentioned above, a new pixel construction that has not conventionally been found is needed to realize the active matrix type electro-optical device having a high pixel aperture ratio with a reduced mask number.
The present invention is made to meet such a request, and an object of the present invention is therefore to provide an electro-optical device having a pixel realizing a high aperture ratio by using a pixel having a novel construction without increasing a mask number and a step number.
To solve the above problems of the prior art, the present invention has the following measures.
In the electro-optical device of the present invention, the attention is paid to the fact that, in the construction of a pixel portion of this device that a certain gate signal line has a constant electric potential in a period except for a period where this gate signal line is selected. The electro-optical device of the present invention is characterized in that, when a gate signal line in an i-th row is selected, one of the gate signal lines including the gate signal line in the i-th row substitutes for an electric current supply line for supplying an electric current to pixels in the i-th row. Thus, it is possible to omit the electric current supply line occupying a not-so-small-area of the pixel portion. High aperture ratio can be realized by this method in the pixel portion without increasing a mask sheet number and a manufacturing step number. Further, if the aperture ratio is set to be equal to the conventional aperture ratio, the width of a signal line can be increased so that resistance and noises can be reduced and image quality can be improved.
According to a first aspect of the present invention, there is provided an electro-optical device comprising a source signal line side driving circuit, a gate signal line side driving circuit and a pixel portion, characterized in that:
the source signal line side driving circuit has a plurality of source signal lines;
the gate signal line side driving circuit has n(n is a natural number, 1 less than n) gate signal lines;
the pixel portion has a structure in which a plurality of pixels are arranged in a matrix-like manner;
the a plurality of pixels controlled by a gate signal line scanned in an i-th column (1xe2x89xa6ixe2x89xa6n) among the n gate signal lines each have a switching transistor, an EL driving transistor, and an EL element;
a gate electrode of the switching transistor is electrically connected to the gate signal line scanned in the i-th column;
one of a source region and a drain region of the switching transistor is electrically connected to the source signal line, and the other is electrically connected to a gate electrode of the EL driving transistor; and
one of a source region and a drain region of the EL driving transistor is electrically connected to one of the n gate signal lines, and the other is electrically connected to one electrode of the EL element.
According to a second aspect of the present invention, there is provided an electro-optical device comprising a source signal line side driving circuit, a gate signal line side driving circuit and a pixel portion, characterized in that:
the source signal line side driving circuit has a plurality of source signal lines;
the gate signal line side driving circuit has n(n is a natural number, 1 less than n) gate signal lines;
the pixel portion has a structure in which a plurality of pixels are arranged in a matrix-like manner;
the a plurality of pixels controlled by a gate signal line scanned in an i-th column (1xe2x89xa6ixe2x89xa6n) among the n gate signal lines respectively have a switching transistor, an EL driving transistor, and an EL element;
a gate electrode of the switching transistor is electrically connected to the gate signal line scanned in the i-th column;
one of a source region and a drain region of the switching transistor is electrically connected to the source signal line, and the other is electrically connected to a gate electrode of the EL driving transistor;
one of a source region and a drain region of the EL driving transistor is electrically connected to one of the n gate signal lines, and the other is electrically connected to one electrode of the EL element; and
an electric current applied to the EL element controlled by the gate signal line scanned in the i-th column is supplied through one gate signal line among the n gate signal lines electrically connected to one of the source region and the drain region of the EL driving transistor.
According to a third aspect of the present invention, there is provided an electro-optical device comprising a source signal line side driving circuit, a gate signal line side driving circuit and a pixel portion, characterized in that:
the source signal line side driving circuit has a plurality of source signal lines;
the gate signal line side driving circuit has n(n is a natural number, 1 less than n)-gate signal lines;
the pixel portion has a structure in which a plurality of pixels are arranged in a matrix-like manner;
the a plurality of pixels controlled by a gate signal line scanned in an i-th column (1xe2x89xa6ixe2x89xa6n) among the n gate signal lines respectively have a switching transistor, an EL driving transistor, and an EL element;
a gate electrode of the switching transistor is electrically connected to the gate signal line scanned in the i-th column;
one of a source region and a drain region of the switching transistor is electrically connected to the source signal line, and the other is electrically connected to a gate electrode of the EL driving transistor;
one of a source region and a drain region of the EL driving transistor is electrically connected to one of the n gate signal lines, and the other is electrically connected to one electrode of the EL element;
the gate signal line scanned in the i-th column has a function for controlling operations of the a plurality of pixels electrically connected to the gate signal line scanned in the i-th column when scanning the i-th column; and
when scanning the gate signal line for controlling the operations of the a plurality of pixels including a plurality of EL driving transistors of which one of the source region and the drain region is electrically connected to the gate signal line in the i-th column, the gate signal line has a function as an electric current supply line for the EL element arranged in each of the a plurality of pixels controlled by the gate signal line.
According to a fourth aspect of the present invention, the electro-optical device of any one of the first to third aspects of the invention is characterized in that:
polarity of the EL driving transistor electrically connected to the EL element is p-channel type when a light emitting direction of the EL element is a direction directed to a substrate on which a driving circuit is formed;
the polarity of the EL driving transistor electrically connected to the EL element is n-channel type when the light emitting direction of the EL element is a direction reverse to the direction directed to the substrate on which the driving circuit is formed; and
polarity of the switching transistor is the same as the polarity of the EL driving transistor.
According to a fifth aspect of the present invention, the electro-optical device of any one of first to fourth aspects of the invention is characterized in that the gate signal line is formed by using aluminum or a material having aluminum as a principal component.